1. Field of the Invention
The invention relates to a motor control unit and a vehicle steering system.
2. Description of Related Art
A motor control unit that controls a brushless motor used in, for example, a vehicle steering system has the function of detecting a current-carrying failure, such as a wire disconnection, in an electric power supply path to the motor. Generally, such a current-carrying failure is detected in the following manner. First, an attempt to apply electric current to the motor is made. Then, if a detected actual current value indicates a non-current carrying state despite the attempt to apply electric current to the motor, it is determined that a current-carrying failure has occurred. Whether an attempt to apply electric current to the motor is being made is determined based on, for example, a current command value or a voltage command value (DUTY command value).
However, even when no current-carrying failure has occurred, if the motor is rotating at a high speed, a high inductive voltage is generated, which may prevent application of electric current to the motor. Therefore, according to, for example, Japanese Patent Application Publication No. 2007-244028 (JP 2007-244028 A), a motor rotation angular velocity is added to determination conditions. In this way, it is possible to prevent a problem that an erroneous determination that a current-carrying failure has occurred is made although no current-carrying failure has actually occurred during high-speed rotation. Thus, it is possible to improve the detection accuracy.
In many cases, a brushless motor is controlled based on a motor rotation angle detected by a rotation angle sensor (resolver). In recent years, however, there has been proposed a motor control unit that controls a motor based on an estimated motor rotation angle without using a rotation angle sensor, as described in Japanese Patent Application Publication No. 2011-51537 (JP 2011-51537 A). In such a motor control unit that controls the motor without using a rotation angle sensor, a motor rotation angular velocity is estimated based on, for example, an inductive voltage value calculated according to a known motor voltage equation based on phase current values and phase voltage values.
However, in a state where a current-carrying failure has occurred in an electric power supply path, the resistance value of the electric power supply path is infinite in theory. Therefore, it is not possible to calculate an appropriate inductive voltage value. Consequently, it is not possible to estimate a motor rotation angular velocity. Accordingly, in the motor control unit that controls the motor without using a rotation angle sensor, it is not possible to use a motor rotation angular velocity to determine whether a current-carrying failure has occurred. Therefore, it is difficult to improve the accuracy of detecting a current-carrying failure, and there is still room for improvement in the detection accuracy.